For vaginal transmission, viruses must penetrate mucus secretions to reach target cells; we recentiy found that Herpes Simplex Virus (HSV) and HIV readily diffuses through human cervicovaginal mucus. Most antibodies (Ab) produced by the immune system are secreted into mucus (not blood or lymph), and topical IgG provides robust protection against vaginal viral challenges. In addition to well-known antibody functions (e.g., neutralization, complement activation, opsonization), an important yet little recognized effector function of IgG is to trap viruses in mucus. IgG bound to a virus surface may form multiple low-affinity adhesive crosslinks between the virus and the mucus gel. A sufficient number of these low-affinity crosslinks, possibly at sub-neutralizing IgG concentrations, may permanently trap the virus in the mucus gel. Our pilot observations indicate that remarkably low concentrations of specific IgGI can trap HSV-1 and virus-like particles that otherwise rapidly penetrate mucus gels. Trapping reduces the flux of virus that reaches target cells, and facilitates inactivation and clearance by additional protective mechanisms. Trapping viruses in mucus before they can reach target cells is likely essential to protect against viruses that, once established, cause incurable infections. It is likely that the most protective antibodies will be those that not only neutralize virions by blocking viral entry into target cells, but also are highly effective at trapping virions in mucus. This potential IgG trapping function in mucus has been largely unrecognized because most studies of IgG activity have not been performed in mucus geis. Hence, all monoclonal antibodies (MAbs) developed to date has been developed without considering this immune function in mucus.